In a process of manufacturing liquid crystal displays, substrates need to be moved or carried from one worktable to another. The substrates are usually held and carried by a substrate box, and on the substrate box, the substrates are loaded, unloaded and picked up by a mechanical arm.
When picking up the substrates, the mechanical arm is usually controlled by a control unit to start picking up the lowest substrate in the substrate box. A position sensor is arranged at a side of the mechanical arm close to the substrates, for sensing the position of the mechanical arm and notifying the control unit of the sensed position, so that the control unit is able to control the mechanical arm to pick up the substrates.
At least the following problem exists in the prior art: when the position of the mechanical arm sensed by the sensor is incorrect, the mechanical arm will collide with the substrates, resulting in breakage of the substrates and/or damage to the mechanical arm.